Charged particle beam apparatus have many functions in a plurality of industrial fields, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for structuring and inspecting specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring, is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices, such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams due to their short wavelengths.
When directing a charged particle beam onto a specimen, depending on the type, the energy, and the impinging direction of the charged particles, a plurality of interactions may occur between the charged particles and the material, in particular the surface of the specimen. These interactions may result in the emission of particles such as electrons at the place of interaction. Generally, for the following discussion, there is no need for distinguishing between secondary electrons, backscattered electrons and Auger electrons. For the purpose of simplicity, these three types of electrons will be referred to as “secondary electrons”. In inspecting applications, the secondary electrons are registered at a detector which is coupled to some means for processing the information received by the secondary electrons.
However, there are, inter alia, two major problems in the state of the art that arise when directing a charged particle beam onto a specimen:
First of all, especially when working with an insulating sample, the sample is charged by the charged particle beam. Typically, in the case of electrons as charged particles, the sample becomes negatively charged whereas in the case of ions as charged particles, the sample becomes positively charged. However, it is possible that the specimen becomes negatively charged by an ion beam or positively charged by a electron beam. The charging is also dependent on the material of the specimen, the charged particle beam energy, and the inclination of the specimen regarding the imaging primary particle beam. The more insulating the material is, the more charge gathers in the sample.
The second major problem is the contamination. In general, the charged particle beam splits hydrocarbon molecules that are present in the vacuum and deposits a carbon layer on the sample and the detector. This contamination damages the detector and/or the sample and results in a bad imaging quality. In particular, in electron beam inspection the detector has to accept several tens or hundreds of nano-ampere detection currents which form carbon layers on the detector and reduce the lifetime. Typically, applied detectors in electron beam inspection are pin diodes whose sensitivity is largely influenced by carbon layers.
In the state of the art, there are several methods known to drain the charging of a specimen. For example, DE 33 32 248 A1 teaches to direct a gas flow onto the specimen surface. Due to the interaction with the charged particle beam, the gas molecules are ionized into positive ions and electrons. As the electrons have low energy, they are rejected by the negatively charged specimen surface, whereas the positively charged ions are attracted by the specimen where they absorb electrons from the specimen's surface. Thereafter, the now uncharged gas molecules are pumped away from the specimen surface.
U.S. Pat. No. 6,555,815 B2 describes a method wherein inert gases such as N2, CO2, SF6, or noble gases are injected onto the sample's surface. According to WO 98/32153 an inert gas is injected into the scanning electron microscope at the point where the electron beam impinges the specimen to neutralize a charge build-up on the specimen by the ionization of the inert gas by the electron beam. Further, WO 98/32153 teaches to flood the scan region with positive charge for a number of frame cycles between scan frames, thereby reducing the positive charged build-up on the specimen. An apparatus for particle beam induced modification of a specimen is described in U.S. Pat. No. 6,182,605. Therein, it is advised to supply a gas, such as Dimethyl-gold-trifluoro-acetylacetonate, in the modification area of the specimen which creates a gas atmosphere in the beam interaction area of the specimen. Due to the interaction of the particle beam with the gas molecules, chemically active atoms and radicals will be generated, which can interact with the specimen in the area of the beam interaction.
In order to overcome the contamination problem described before, U.S. Pat. No. 5,981,960 teaches a method and apparatus wherein ozone gas is introduced into the chamber through which the charged particle beam is passed, shaped and deflected. The gas is supposed to be irradiated to the desired location while the charged particle beam is irradiated through the chamber. A charge-up drift due to a contamination material from a resist on a wafer can be avoided by the ozone self cleaning. Furthermore, U.S. Pat. No. 5,312,519 and U.S. Pat. No. 5,466,942 disclose methods of cleaning a charged beam apparatus wherein ozone is introduced into the chambers of the charged particle beam apparatus. The problem is, however, that the cathodes are generally very sensitive to gas, in particular to oxygen exposure. Hence, the ozone must be prevented from coming into contact with the cathodes. This problem is partly overcome e.g. by the U.S. Pat. No. 5,981,960 by providing the charged particle beam apparatus with several chambers.
The problems in the state of the art are especially challenging in high current density, low voltage electron beam systems used e.g. in electron beam wafer and mask inspection and metrology. In those applications, the specimen is rapidly charged due to the high current. Additionally, due to the low voltage, in comparison to high voltage beam systems, the charging is relatively large thus redirecting the charged particles. This, in turn, results in a bad imaging quality. Moreover, the high current low voltage probe is generated by thermal field emission (Schottky) or cold filed emission cathodes. These cathode types, however, are very sensitive to gas, in particular to oxygen exposure.
Accordingly, it is an object of the present invention to overcome at least part of the problems in the state of the art. It is particularly an object of the present to provide a charged particle beam apparatus and a method of operating thereof that reduces the charging and/or contamination effects.